Turbomachines such as steam turbines are exposed to a throughflow of a flow medium which as a rule has high temperatures and pressures. Therefore, in a steam turbine as an embodiment of a turbomachine steam is used as the flow medium. The steam parameters in the live steam inlet region are high to such an extent that the steam turbine is thermally heavily stressed at various points. Therefore, for example in the inlet region of the steam turbine the materials are thermally heavily stressed. A steam turbine comprises in the main a turbine shaft, which is rotatably mounted, and also a casing which is arranged around the turbine shaft. The turbine shaft is thermally heavily stressed as a result of the temperature of the inflowing steam. It is accepted that the higher the temperature, the higher is the thermal stress. Turbine blades are arranged on the rotor in so-called slots. During operation, the slots experience a high level of mechanical stress. The thermal stress, however, lowers the tolerable mechanical stress as a result of rotation and additional loading by the blades which are fastened on the rotor.
From the thermodynamic point of view, it makes sense to raise the inlet temperature of the steam since the efficiency increases with higher inlet temperature. In order to extend the load capacity of the materials used in the steam turbine at high temperatures, the inlet regions of the shaft are cooled. Providing a suitable cooling method can be developed, changing to a higher quality, but more expensive, material can be dispensed with.
A steam turbine plant comprises at least one steam generator and a first steam turbine, which is designed as a high-pressure turbine section, and further turbine sections which are designed as an intermediate-pressure turbine section or a low-pressure turbine section. After live steam has flown through the high-pressure turbine section, the steam is heated again in a reheater to a high temperature and conducted into the intermediate-pressure turbine section. The steam which comes from the high-pressure turbine section is referred to as cold reheat steam and is comparatively cool in comparison to the live steam. This cool reheat steam is used as cooling medium.
This means that the cold reheat steam is conducted into the inlet region of the steam turbine and lowers the material temperature there. However, it is such that the cold reheat steam in the inlet region, for example in an intermediate-pressure turbine section, leads to very large temperature differences. This leads to the disadvantage that despite the cooling locally high temperature gradients, and high thermal stresses as a result thereof, occur. Furthermore, it can bring about local dimensional changes which is enforced by thermal distortion as a result of unequal thermal expansion since intensely cooled and uncooled regions are arranged next to each other. Furthermore, in the event of a cooling failure, i.e. that the cold reheat steam is not made available and therefore forms a failure case, thermal shocks occur, leading to extremely severe thermal stresses.
In the failure case, this means that in the event of a failure of the cooling the previously cooled shaft expands to a significant degree. This thermal expansion is structurally to be taken into consideration and makes the conducting of the cooling medium and sealing of the cooled region more difficult.
Document DE 34 06 071 A1 disclosed a shield, wherein the shield has only a cooling steam line but no additional line.